By exposing the core of a photosensitive optical fibre to a UV laser interference pattern, its refractive index is changed periodically, thereby producing a fibre Bragg grating (FBG). The photosensitivity of an optical fibre may be increased by hydrogenating the fibre. In this specification hydrogenation of an optical fibre is understood to mean loading an optical fibre with hydrogen or an isotope of hydrogen, such as deuterium.
A number of FBG types have been distinguished, characterised by markedly different spectral and thermal behaviours, originating from the UV excitation of several distinct physical mechanisms. During the typical inscription process, the resonant wavelength of the grating increases with UV exposure duration, indicative of the induction of a positive change in refractive index. The resulting FBG has come to be referred to as a Type I grating, and is the type most commonly fabricated using either free-space holographic or phase mask exposure techniques. However, at high temperatures this FBG is the least stable of the grating types reported to date.
With further UV exposure, the Type I grating growth becomes saturated, and formation of the so-called Type IIA grating commences. The observed reflectivity decreases almost to zero before increasing again and, eventually, saturating. The resonant wavelength of the grating decreases during this stage of the exposure, indicating a negative induced refractive index change. Type IIA gratings are usually inscribed at a wavelength of 193 nm and although the underlying mechanism is still not wholly clarified, links to stress and defect concentrations have been identified.